DeskClock: Failed to set up Alarm

1. Add alarm will prompt error tips because it does NOT use
Notification Channel while NotificationCompat.Builder.build
2. Failed to start Alarm notification because it can NOT start
background service in Andorid O while APP is in Idle mode

Set targetSdkVersion as 23 so the AlarmManager/Notification
API will keep behavior as Android N

Change-Id: I3624363ffb9143fb8835cb9b9d4ff967748fa415
CRs-Fixed: 2048293